Effect of Mineralogical Changes on Mechanical Properties of Well Cement

ARMA 19–1981

Well integrity of high temperature wells: Effect of mineralogical changes on mechanical properties of well cement

TerHeege, J.H. and Wollenweber, J. TNO Applied Geosciences, Utrecht, the Netherlands

Marcel Naumann Downloaded from http://onepetro.org/ARMAUSRMS/proceedings-pdf/ARMA19/All-ARMA19/ARMA-2019-1981/1133466/arma-2019-1981.pdf/1 by guest on 02 October 2021 Equinor ASA, Sandsli, Norway Pipilikaki, P. TNO Structural Reliability, Delft, the Netherlands Vercauteren, F. TNO Material Solutions, Eindhoven, the Netherlands

This paper w as prepared for presentation at the 53rd US Rock Mechanics/Geomechanics Symposium held in New Y ork, NY , USA , 23–26 June 2019. This paper w as selected for presentation at the symposium by an ARMA Technical Program Committee based on a technical and critical review of the paper by a minimum of tw o technical reviewers. The material, as presented, does not necessarily reflect any position of ARMA, its of ficers, or members.

ABSTRACT: Wells used for steam-assisted gravity drainage (SAGD), for cyclic steam stimulation (CSS), for hydrocarbon production in areas with anomalous high geothermal gradient, or for geothermal energy extraction all are operated in high temperature e nvironments where maintaining long term wellbore integrity is one of the key challenges. Changes in cement mineralogy and associated mechanical properties may critically affect the integrity of high temperature wells. In this study, the relation between changes in mineralogy and mechanical properties of API class G cement with 40% silica flour was investigated by exposing samples for 1-4 weeks to temperatures of 60-420°C. The effect of mineralogical changes on mechanical properties was investigated using a combination of chemical and microstructural analysis and triaxial strength tests at confining pressures of 2-15 MPa. The combined effect of changing porosity, microstructure and strength of mineral phases determines changes in mechanical behavior, in particular the formation of stronger mineral phases with higher density affect Young’s modulus, failure strength and residual strength. Deformation of the cement is more brittle at low confining pressures and more ductile at high confining pressures with a considerable residual strength after failure and highest reduction in cement stiffness and increase in strength at temperatures of ~250°C.

high pressure (HP) wells (Enform, 2012; Chartier et al., 2015). For standard curing conditions, hydration of 1. INTRODUCTION ordinary Portland cement (OPC, e.g., class G well Wells that are drilled or operated in high temperature cement) with a calcium oxide to silicon dioxide (C/S) environments can be subject to higher risks of losing ratio close to 3.1 produces the amorphous C-S-H and the integrity and zonal isolation. Examples of such high crystalline Ca(OH)2. The product is a mixture of ordered temperature wells include wells used for steam-assisted and disordered phases with C-S-H present as a highly gravity drainage (SAGD) or for cyclic steam stimulation disordered version of the crystalline tobermorite and (CSS), wells drilled in areas with anomalous high jennite phases (Taylor, 1990). Tobermorite preserves high geothermal gradient (HPHT wells), or geothermal wells compressive strength and low permeability. (Stiles, 2006; Chartier et al., 2015; Frioleiffson et al., Mineralogical and mechanical changes are expected to 2015). Maintaining long term wellbore integrity in these occur in OPC above ~110°C (Thorvaldson et al., 1938; situations is one of the key challenges determining the Patchen, 1960; Taylor, 1990; Kyritsis et al., 2009). technical and commercial success of projects that require Reactions result in different hydration products, i.e. wells to operate at high temperatures. crystallization of alpha dicalcium silicate hydrate (α- C SH or Ca SiO (OH) ) is favored. The lime-rich alpha- Changes in cement mineralogy and associated mechanical 2 2 3 2 dicalcium silicate hydrate (α-C SH) is denser than the properties critically affect the integrity of high 2 temperature wells. Standard cement formulations have usual calcium silicate hydrate (C-S-H gel) product. Bulk been reported to reach their mechanical and chemical volume decrease, and cement shrinkage or porosity increase is associated with this reaction, which results in (stability) limits due to the harsh conditions and extreme a decrease in cement strength and increase in permeability cyclic temperature loads in high temperature (HT) and (Patchen, 1960; Pernites and Santra, 2016). All types of findings aid in assessing risks of well integrity loss and OPC and other types of well cements with simila r mitigation of these risks by designing new cement composition show lower strengths when cured above formulations. The main challenge remains to optimize 110°C, compared to curing at 93°C. cement properties for prolonging the integrity of cement at high temperatures while maintaining low viscosity of Elevated curing temperatures are commonly reached by cement slurry during well cementation. SAGD, CCS, HPHT oil and gas wells or geothermal wells in areas with moderate to high geothermal gradients. Changes in cement properties need to be accounted for in 2. EXPERIMENTAL APPROACH the design of cement formulations as well as cementation 2.1. Sample material procedures of these wells to ensure the plugging of loss Downloaded from http://onepetro.org/ARMAUSRMS/proceedings-pdf/ARMA19/All-ARMA19/ARMA-2019-1981/1133466/arma-2019-1981.pdf/1 by guest on 02 October 2021 zones during drilling, proper anchoring, corrosion Synthetic cement samples, Dyckerhoff HT Basic Blend of protection of casing strings, zonal isolation and well cement clinker (API class G cement) with 40 % silica integrity at elevated temperatures and pressures (HPHT flour was used as a starting material (Papaioannou, 2018). conditions). A common practice is to use Portland-type Specimens are prepared using the Dyckerhoff blend and cements with added silica (such as fly ash, silica fume or demineralized water with a water to solid (cement + silica quartz flour) to stabilize cement at higher temperature. flour ) ratio of 0.371 (m/m). For the mixing process, the EN 196-3 protocol was followed. After each step the Reactions of Ca(OH)2 with added silica at elevated temperatures result in a C-S-H with a lower C/S ratio. weight of the samples was checked for dehydration. After Addition of 35 to 40% by weight of cement is enough to mixing, the slurry was injected into cylindrical molds with 25 mm internal diameter and 60 mm length using a prevent the reaction forming α-C2SH. Instead, cement phases such as tobermorite or xonotlite are formed 10 ml syringe in order to achieve homogeneity and (Patchen, 1960). Moreover, for a water to solids ratio of minimize air entrapment. The molds were subsequently 0.4, a formulation with added silica results in 91% of the sealed to prevent water evaporation, and exposed for 72 water being chemically bound against only 39 % of the hours at 60°C and atmospheric pressure in a furnace. available water in the OPC. Addition of ~40% silica flour Samples exposed to 120, 250 and 420°C were thereby favors formation of high strength and low subsequently cooled down for 2 hours before demolding. permeability mineral phases. Pressure resistant glass bottles were used for exposure of pairs of samples under suspension of water at 120°C and Despite the importance for well integrity of high 2 bar in a furnace. Metal sample holders sealed by a cap temperature wells, data on mineralogical and mechanical with two different types of O-rings were used with a pre- changes during exposure of well cements to HPHT defined amount of water to obtain required exposure conditions is limited. Some relevant laboratory studies are conditions. Samples exposed to 60 and 120°C were available (Taylor 1990; Gabrovšek et al., 1993; Meller et cooled down in two hours, while for samples exposed to al., 2009; Pernites and Santra, 2016), but investigations of 250 and 420°C samples a 3 day gradual cooling protocol the relation between mineralogical and mechanical was applied. For mineralogical and microstructural changes at different exposure times and temperatures are analysis, samples were exposed for 4 weeks to 60, 120, sparse. In this study, the relation between changes in 250 and 420°C. For the triaxial experiments, samples mineralogy and mechanical properties of API class G were cut and manually polished to obtain samples with cement with 40% silica flour was investigated. Samples plane parallel surfaces, a final length between 49.6 and were exposed to temperatures of 60°C (curing only) up to 52.2 mm, and a final diameter between 24.6 and 25.0 mm. 420°C (curing and exposure) for exposure times of 3 days Samples were tested after curing (“cured only” samples), (curing time) to 4 weeks (HT exposure). The effect of or after exposure of 1 week to 120°C at 2 bars, 4 weeks to mineralogical changes on mechanical properties was 250°C at 40 bars, and 2 weeks to 420°C at 350 bars, investigated using a combination of (1) chemical analysis respectively. using X-ray diffraction (XRD), (2) microstructural 2.2. Mineralogical and microstructural analysis analysis using scanning electron microscopy (SEM) and optical microscopy, and (3) mechanical properties Analysis of mineral phases in the samples was performed (Young’s modulus, failure stress, residual stress) using using X-ray diffraction (XRD) and energy dispersive X- ray spectroscopy (EDX). Microstructural analysis was triaxial tests at confining pressures of 2-15 MPa. performed using optical microscopy and scanning Results show that the formation of high temperature electron microscopy (SEM). mineral phases in the cement such as tobermorite, xonotlite and wollastonite is associated with marked XRD analysis was performed on grinded aliquots to changes in elastic properties and failure behavior. Critica l access the crystalline compounds and deterioration conditions for the changes in mechanical behavior are products. Quantitative analysis was performed on determined, and the implications for well integrity and samples exposed for 4 weeks to 60, 120 and 420°C (2 samples, Table 1). Mineralogy of samples exposed for 4 design of new cement formulations are discussed. The weeks to 250°C was analyzed on the basis of a XRD shortening of the piston assembly is recorded. Therefore, spectrum of one sample. Samples were measured in axial strains measured in this setup are likely Bragg-Brentano geometry with a Bruker D8 Advance overestimated, possibly leading to an apparent lower diffractometer, equipped with a motorized slit with sample stiffness (Young’s modulus). Radial strains were opening angle of 0.30°, primary and secondary soller slit not measured. Axial stress was measured by two load of 2.5° and Lynxeye detector with opening angle of 2.94° cells, located both inside and outside the pressure vessel. and Ni-Kß filter. Data collection was carried out at room Triaxial tests were conducted by hydrostatic loading of temperature using Cu Kα radiation (λ = 0.15406 nm) in samples to predefined confinement, and subsequent the 2θ region between 10° and 120°, step size 0.015 triaxial compression at constant displacement rate until degrees 2θ. The sample was deposited on a zero

reaching an approximately constant plateau of post- Downloaded from http://onepetro.org/ARMAUSRMS/proceedings-pdf/ARMA19/All-ARMA19/ARMA-2019-1981/1133466/arma-2019-1981.pdf/1 by guest on 02 October 2021 background holder (Si single crystal <510> wafer) and failure (residual) stress. In two tests (GWL005 and was rotated at 15 rpm during the measurement. Generator GWL007), triaxial compression at constant displacement settings were 40 kV and 40 mA. rate was initially to an axial stress of about 1/3 to 1/2 of For optical microscopy analysis, thin sections of the expected peak stress, followed by triaxial unloading to cement samples were prepared by sawing specimens of initial stress conditions and re-loading until reaching an 50 × 30 × 15 mm (length × width × thickness) from each approximately constant plateau of post-failure (residual) of the samples. The sawn specimens were slowly dried at stress at the same displacement rate. 20ºC and subsequently impregnated under vacuum with The triaxial experiments were used to determine an epoxy resin containing a fluorescent dye. After mechanical parameters (static Young’s modulus, failure hardening of the resin, a thin section with a thickness of and residual strength) as function of stress state µ about 25 m was prepared prism by grinding and (confinement), specimen exposure temperature and polishing. Impregnation of the specimens with a duration. fluorescent resin allows thin sections to be studied with both transmitted-light and fluorescent-light microscopy. The thin sections were examined with a LEICA optical 3. RESULTS microscope. 3.1. Mineralogy The SEM used in this investigation is a FEI Samples cured for 4 weeks at 60°C mainly consist of NovaNanoSEM 650. The SEM was operated at an hydration products ettringite, portlandite and katoite, acceleration voltage of 5 kV in low (40 Pa) vacuum mode amorphous hydrate phase and clinker phases C3S and to avoid charging of the non-conductive specimen. The brownmillerite that have not completely reacted (Table gaseous analytical solid-state, back-scattered electron 1). detector (GAD) was used for imaging both topographic and material contrast. The same thin sections as used for Table 1. Mineralogical composition of samples exposed for 4 optical microscopy were analyzed. The SEM is equipped weeks at different temperatures determined by XRD analysis. with a Noran micro analysis system (EDX) for analyzing chemical composition. EDX analysis was performed on Mineral phase 60°C 120°C 420°C* bulk material and specimens with a size down to Ettringite 1.4 - - approximately 1 µm. Portlandite 8.5 - - Katoite 3.5 2.6 - 2.3. Triaxial experiments C3S 3.8 - - The triaxial experiments were conducted at confining Brownmillerite 7.7 5.0 - pressures of 2-15 MPa, room temperature and pore Tobermorite - 10.4 - pressures at ambient conditions. Samples were tested at Afwillite - 0.6 - initially fully saturated pore fluid conditions. Slow axial α-C2S hydrate - 1.2 - strain rate on the order of 5.10-6 s-1 were used to remain in Quartz 26.2 13.1 - drained conditions such that no excessive pore pressure is Xonotlite - - 33.8-35.3 built-up internally. The triaxial cell allows for Wollastonite - - 14.8-15.9 independent control of axial and radial stresses Gypsum - - 1.1-2.4 (confinement). Confining pressure is controlled via an Calcite - - 0.3 external pump. The setup and procedure follow largely Amorphous content 48.9 67.1 46.1-49.9 *Range from measurement of two different samples. the suggested method for determining the strength of rock materials in triaxial compression (Kovari et al., 1983). Samples exposed for 4 weeks to 120°C mainly consist of Axial strains were measured indirectly by means of an hydration phases tobermorite, α-C S hydrate and external linear variable differential transformer (LVDT) 2 afwillite. The clinker phase C S as well as the hydration only, implying that the combined displacement due to 3 products ettringite and portlandite have disappeared both sample deformation and load-dependent elastic completely at these conditions, while brownmillerite and calcite and gypsum are also formed. The amorphous katoite are detected in smaller amounts. Dissolution of content decreased again compared to the samples exposed quartz occurs, but reactions involving quartz are limited. for 4 weeks at lower temperatures (Table 1). The amount of metastable high Ca/Si ratio amorphous phases increases. Xonotlite is the main mineral phase that 3.2. Microstructure appears in samples exposed for 4 weeks to 250oC, Microstructures of samples cured for 4 weeks at 60°C accompanied by small amount of calcite. All other phases show a very fine grained to amorphous matrix with areas including cement hydrates, quartz and non-hydrated of high and low porosity and large inhomogeneity of pore structure and water distribution in the sample (Fig. 1). cement have disappeared. Samples exposed for 4 weeks Both darker areas with dense C-S-H cement grains and to 420°C still mainly consist of xonotlite, but wollastonite, Downloaded from http://onepetro.org/ARMAUSRMS/proceedings-pdf/ARMA19/All-ARMA19/ARMA-2019-1981/1133466/arma-2019-1981.pdf/1 by guest on 02 October 2021 60°C 60°C

300 µm 10 µm 120°C 120°C

2 µm 600 µm

420°C 420°C

50 µm 5 µm

Fig. 1. Microstructures of samples exposed for 4 weeks to 60-420°C. Upper and middle left- light microscopy, others- scanning electron microscopy. See text for description of phases.

lower capillary porosity, and lighter areas with higher Table 2. Mechanical data from triaxial experiments. capillary porosity are present indicating local separation Sample Exposure Pc σp σr E of water and micro-bleeding. The microstructure and MPa MPa MPa GPa distribution of capillary porosity of samples cured for 4 GWL003 Cured only 2 49.3 19.3 4.9 weeks at 120°C is more homogeneous than in the samples GWL005 Cured only 2 57.4 33.1 8.2 exposed to 60°C with similar porosity throughout the GWL011 Cured only 2 58.1 28.2 7.2 sample. A reaction front is observed between the C-S-H GWL004 Cured only 5 61.4 38.5 5.8 gel and the surrounding amorphous silica, and well- GWL002 Cured only 10 69.1 54.5 7.8 formed tobermorite crystals appear to grow inside the C- GWL009 Cured only 10 61.5 17.6 6.5 S-H gel. The microstructure of samples cured for 4 weeks GWL006 Cured only 15 80.1 75.1 9.5 at 420°C show needle-shaped crystals (probably GWL012 1 week, 120°C 2 52.3 32.9 6.4 Downloaded from http://onepetro.org/ARMAUSRMS/proceedings-pdf/ARMA19/All-ARMA19/ARMA-2019-1981/1133466/arma-2019-1981.pdf/1 by guest on 02 October 2021 wollastonite) cover the whole sample with longer crystals GWL013 1 week, 120°C 5 54.9 38.7 7.0 forming inside pre-existing voids and pores causing a GWL007 1 week, 120°C 10 75.1 56.3 7.5 reduction of porosity. Other (mainly amorphous phases) GWL018 4 weeks, 250°C 2 29.5 18.3 4.8 contain a relatively high amount of Fe, causing a reddish GWL014 4 weeks, 250°C 5 31.3 26.4 3.9 discoloring in some samples. GWL019 4 weeks, 250°C 5 34.5 31.2 4.6 3.3. Mechanical behavior GWL016 2 weeks, 420°C 2 57.6 29.3 6.7 GWL015 ° 5 48.2 31.5 5.6 Cured only samples have a Young’s modulus (E) that 2 weeks, 420 C GWL017 ° 10 58.0 47.1 5.8 increases from 4.9 GPa at 2 MPa confining pressure (P ) 2 weeks, 420 C c to 9.5 MPa at Pc = 15 MPa (Table 2, Figure 2). A simila r increasing trend of Young’s modulus with confining For longer exposures or higher exposure temperatures pressure is observed for samples exposed for 1 week to Young’s modulus is mostly decreasing with increasing 120°C (E = 6.4 GPa for Pc = 2 MPa to E = 7.5 GPa for Pc exposure time and exposure temperature. One exception = 10 MPa). Compared to cured only samples, Young’s is a sample exposed for 2 weeks to 420°C tested at Pc = 2 modulus is increasing for Pc = 2 and 5 MPa, and MPa, which shows a higher Young’s modulus compared decreasing for Pc = 10 MPa. The trend of increasing to cured only samples or samples exposed to 120°C. Young’s modulus with increasing confining pressure is σ reversed for samples exposed for 4 weeks to 250°C and The maximum (peak) differential stress ( p or failure for 2 weeks to 420°C. Young’s modulus is increasing if strength) and (residual) differential stress after failure (σr samples are exposed for 1 week to 120°C, at least for low or residual strength) increases with confining pressure in confining pressures. almost all samples (Table 2, Fig. 3). Failure strengths of

10.0 15 MPa curing only 9.0 1 week, 120°C 4 weeks, 250°C 8.0 10 MPa 10 MPa 2 weeks, 420°C 7.0 5 MPa 2 MPa color shades/ 2 MPa 6.0 labels indicate Pc 5 MPa 5 MPa 10 MPa 5.0 2 MPa 2 MPa 5 MPa 4.0

Young's modulus Young's [GPa] 3.0

2.0

1.0

0.0 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5

exposure time [weeks] Fig. 2. Young’s modulus of “cured only” samples and samples exposed for 1-4 weeks to 120-420°C.

90.0 curing only 80.0 15 MPa 1 week, 120°C 10 MPa 4 weeks, 250°C 70.0 10 MPa 2 weeks, 420°C residual stress 60.0 5 MPa 2/10 MPa 5 MPa color shades/ 50.0 2 MPa 2 MPa 5 MPa labels indicate Pc Downloaded from http://onepetro.org/ARMAUSRMS/proceedings-pdf/ARMA19/All-ARMA19/ARMA-2019-1981/1133466/arma-2019-1981.pdf/1 by guest on 02 October 2021 40.0 5 MPa 30.0 2 MPa failure/residual stress [MPa] 20.0

10.0

0.0 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5

exposure time [weeks] Fig. 3. Failure and residual strength of “cured only” samples and samples exposed for 1-4 weeks to 120-420°C.

the cured only samples are 49.3-58.1 MPa at Pc = 2 MPa these effects (for examples samples exposed for 2 weeks to 80.1 MPa at Pc = 15 MPa. One sample exposed for 2 to 250°C or for 4 weeks to 420°C). weeks to 420°C has similar failure strength for Pc = 2 MPa and 10 MPa, but its residual strength shows the general 4. DISCUSSION trend of increasing strength with increasing confining pressure. Trends of failure and residual strength with The relation between mineralogy, microstructure and exposure time and temperature are comparable to the mechanical behavior was investigated for cement samples trends observed for Young’s modulus, i.e. overall failure consisting of Dyckerhoff HT Basic Blend of cement and residual stresses decrease with increasing exposure clinker (API class G cement) with 40% added silica flour. time and exposure temperature. Samples exposed for 1 Previous studies showed that addition of 35 to 40% by week to 120°C have strengths that are comparable to the weight of cement is enough to prevent the reaction cured only samples. The relation between principal forming α-C2SH, and favor formation of stronger mineral stresses (σ1, σ3) at failure (Fig. 4) suggest (1) a similar phases such as tobermorite or xonotlite (Patchen, 1960). friction coefficient for samples exposed up to 250°C, (2) Addition of ~40% silica flour is therefore expected to a change in friction coefficient with increasing exposure favor formation of high strength and low permeability temperature between 250 and 420°C, and (3) a drop in mineral phases at high temperatures, thereby improving uniaxial compressive strength (UCS) with exposure cement stability and well integrity in high temperature temperatures increasing from 120 to 250°C, followed by wells. an increasing UCS between 250 and 420°C. Note that σ1 The current study shows that deformation of the cement = C0 + qσ3, with C0 the uniaxial compressive strength and is more brittle deformation at low confining pressures and q related to the friction coefficient for a Mohr-Coulomb more ductile deformation at high confining pressures. The failure criterion (Al-Ajm and Zimmerman, 2005). The relation between mineralogy, microstructure and difference between failure and residual strength (Fig. 5) mechanical behavior is complex. Mineralogical changes suggest a change from more brittle deformation at low occurring between curing at 60 and 120°C mostly result confining pressures (i.e. a large stress drop after failure or in slightly stiffer (i.e. higher Young’s modulus) and large difference in strength) to more ductile deformation stronger (i.e. higher failure and residual strength) cement. at high confining pressures (i.e. a small stress drop after Cement becomes less stiff and weaker if temperatures are failure that is spread out over larger axial strain). raised to 250°C. It becomes stiffer and stronger again if The relative effect of exposure time and temperature on temperatures are further increased to 420°C. The mechanical behavior is unclear. Further experiments on observed changes may also be affected by exposure times, samples need to be performed to systematically explore which have not been systematically varied. 100

70 [MPa] 1 σ 60 Downloaded from http://onepetro.org/ARMAUSRMS/proceedings-pdf/ARMA19/All-ARMA19/ARMA-2019-1981/1133466/arma-2019-1981.pdf/1 by guest on 02 October 2021 50

30 cured only axial stress at failure 20 1 week, 120°C 4 weeks, 250°C 10 2 weeks, 420°C 0 0 2 4 6 8 10 12 14 16

confining pressure σ3 [MPa]

Fig. 4. Relation between the maximum principal stress (σ1) at failure and minimum principal stress (σ3 = Pc) of “cured only” samples and samples exposed for 1-4 weeks to 120-420°C. Best fit linear trends are also indicated (dashed lines).

100

70 [MPa] 1 σ 60

30 cured only axial stress at failure 20 1 week, 120°C 4 weeks, 250°C 10 2 weeks, 420°C 0 0 2 4 6 8 10 12 14 16 σ confining pressure 3 [MPa]

Fig. 5. Relation between the maximum principal stress (σ1) at post failure residual strength and minimum principal stress (σ3 = Pc) of “cured only” samples and samples exposed for 1-4 weeks to 120-420°C. Best fit linear trends are also indicated (dashed lines).

Comparison with other studies (Enform, 2012; isolation of the casing by the cement sheath and enhances Papaioannou 2018) and mineralogical observations (i.e. thermal stresses and corrosion. Higher mechanical loads formation of stronger mineral phases) suggest that (1) on the casing will increase the risks of casing buckling samples may become stiffer and stronger for longer and collapse. For the high temperature cement exposures at 120°C, and (2) stiffness and strength will investigated in this study, a change in the mechanical further increase for longer exposure to 420°C. behavior of the cement is observed from more brittle deformation at low confining pressures to more ductile The change in mechanical behavior is likely due to the deformation at high confining pressures with a combined effect of changing porosity, microstructure and considerable residual strength after exceeding the failure strength of mineral phases, i.e. (1) formation of stronger strength. It suggests that the cement may still support xonotlite and wollastonite phases with higher density at Downloaded from http://onepetro.org/ARMAUSRMS/proceedings-pdf/ARMA19/All-ARMA19/ARMA-2019-1981/1133466/arma-2019-1981.pdf/1 by guest on 02 October 2021 relatively large differential loads under high confinement, the expense of tobermorite, α-C2S hydrate, afwillit e , even after exceeding its failure strength. Accordingly, the brownmillerite and katoite (absent after exposure for 4 experimental data in this study aid in the development ° weeks at 420 C) and change in amorphous content (Table new cement formulations with improved mechanical 1), (2) change in porosity due the different volume of properties and in the design of operations for high reaction products, and (3) different microstructure (i.e. temperature wells to mitigate cement damage. The main voids filled with needle-shaped crystals). challenge in designing improved cement is to mitigat e The most important issue with this complex relation problematic changes in mechanical properties during between mineralogical, microstructural and mechanical exposure at high temperatures while maintaining low changes in this high temperature cement is with the viscosity of cement slurry during well cementation. The reduction in stiffness and strength around ~250°C. data also provide valuable input for modeling studies that Young’s modulus can drop by 25-44% between 120°C aim to assess critical conditions for loss of well integrit y and 250°C within weeks of high temperature exposure, (Kaldal et al., 2016; TerHeege et al., 2018). depending on confining pressure and exposure times (Table 2). This drop in stiffness can be accompanied by a 5. CONCLUSIONS drop in failure strength of ~40%. Accordingly, in this temperature interval, support and protection of the casing The relation between changes in mineralogy and by cement sheath can be significantly reduced, and well mechanical properties of Dyckerhoff HT Basic Blend of integrity may be jeopardized. Operation temperatures of cement clinker (API class G cement) with 40 % silic a 210-240°C are common for SAGD wells (Chartier et al., flour was investigated. Samples were exposed to 2015), while operations temperatures may fluctuate temperatures of 60-420°C for different exposure times. between 25 and 340°C for CCS wells (Stiles, 2006). The effect of mineralogical changes on mechanical Accordingly, operation of these wells is in the properties was investigated using a combination of (1) temperature window for which the reduced stiffness and chemical analysis using X-ray diffraction (XRD), (2) strength is observed, and well integrity will be affected by microstructural analysis using scanning electron these changes in mechanical behavior. Some geothermal microscopy (SEM) and optical microscopy, and (3) wells target super-hot reservoirs containing supercritical mechanical properties (failure and residual strength, fluids, such as the IDDP-1 and IDDP-2 well (Iceland Young’s modulus) using triaxial strength tests at Deep Drilling Project) with envisaged reservoir confining pressures of 2-15 MPa. temperatures and pressures reaching up to above 400°C The following can be concluded: and 300 bars (Friðleifsson et al., 2015). Long term exposure of well cement to temperatures above 400°C is • The relation between mineralogy, microstructure expected for such wells. Wellbore integrity of these wells and mechanical behavior of synthetic cement may be critically affected by changing mechanical samples is complex. Mineralogical changes properties of well cement between 120 and 250°C during occurring between curing at 60 and 120°C result in thermal recovery after drilling. The cement may regain slightly stiffer and stronger cement. Cement stiffness and strength after exposure at temperatures becomes less stiff and weaker if temperatures are above ~400°C, but not if the cement is already severely raised to 250°C, and stiffer and stronger if fractured before reaching these conditions. temperatures increase to 420°C for intact samples. • The change in mechanical behavior is likely due to Important issues with loss of well integrity in high the combined effect of changing porosity, temperature wells are that cement damage leads to (1) microstructure and strength of mineral phases. The fluid migration along the wells (i.e. casing-cement-rock most important mineralogical changes are the interfaces) and through the cement sheath to the casing, formation of stronger xonotlite and wollastonite and (2) reduced support of the casing to sustain phases with higher density at the expense of mechanical loads. Fluid migration reduces thermal tobermorite, α-C2S hydrate, afwillit e , brownmillerite and katoite, and changes in 5. Friðleifsson, G.Ó., B. Pálsson, A.L. Albertsson, B. amorphous content. These mineralogical changes Stefánsson, E. Gunnlaugsson, J. Ketilsson, and Þ. are accompanied by a change in porosity due the Gíslason. 2015. IDDP-1 Drilled Into Magma – World’s different volume of reaction products, and by Firs t Magma-EGS System Created. In Proceedings changes in microstructure (e.g., voids filled with World Geothermal Congress, Melbourne, Australia, 19- 25 April 2015. needle-shaped crystals). • For all exposure times and temperatures, 6. Kaldal, G.S., M.T. Jonsson, H. Palsson, and S.N. deformation of the cement is more brittle Karlsdottir. 2016. Structural modeling of the casings in deformation at low confining pressures and more the IDDP-1 well: Load history analysis. Geothermics 62: 1-11. ductile deformation at high confining pressures. A considerable residual strength remains after 7. Kovari, K., A. Tisa, H.H. Einstein, and J.A. Franklin . Downloaded from http://onepetro.org/ARMAUSRMS/proceedings-pdf/ARMA19/All-ARMA19/ARMA-2019-1981/1133466/arma-2019-1981.pdf/1 by guest on 02 October 2021 exceeding the peak differential stress, and the 1983 Suggested methods for determining the strength of cement may still carry large differential loads even rock materials in triaxial compression: Revised version. Int. J. Rock Mech. Min. Sci. & Geomech. Abstr. 20: 283– after exceeding its failure strength. 290. • The integrity of high temperature wells used for 8. Kyritsis, K., C. Hall, D.P. Bentz, N. Meller, and M.A. steam assisted gravity drainage and cyclic steam Wilson. 2009. Relationship between engineering stimulation is particular affected by the properties, mineralogy, and microstructure in cement- mineralogical, microstructural and mechanical based hydroceramic materials cured at 200 - 350 C. changes as operation temperatures of these wells Journal of the American Ceramic Society 92: 694–701. are in the same temperature window for which 9. Meller, N., C. Hall, and J.S. Phipps. 2005. A new phase reduction in cement stiffness and strength is diagram for the CaO–Al2O3 –SiO2 –H2O hydroceramic highest. Geothermal wells targeting super-hot system at 200°C. Materials Research Bulletin 40: 715– reservoirs containing supercritical fluids may be 723. critically affected by changing mechanical 10. Papaioannou, A. 2018. Impact of in-situ ageing on the properties of well cement between 120 and 250°C mineralogy and mechanical properties of Portland- during thermal recovery after drilling, but the based cement in geothermal applications. 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